Polymers of 2,2-bis(4-hydroxyphenyl)propane (hereinafter referred to as "bisphenol A"), terephthalic acid and isophthalic acid are well known wholly aromatic polyesters, generally being referred to as polyarylates. Polyarylates are commercially available, and their high heat distortion temperatures and transparency make them useful as a substitute for lower melting transparent polymers (e.g., polycarbonates) in high temperature applications. Other transparent polymers that can be used at elevated temperatures are not commonly available.
Polyarylates that also include 4-hydroxybenzoic acid as a comonomer have been described in the literature. These reports suggest that these polymers may have higher heat distortion temperatures than polyarylates without 4-hydroxybenzoic acid, but are difficult to process. For example, U.S. Pat. No. 4,189,549 describes and claims blends of the copolymers of isophthalic and/or terephthalic acid, 4-hydroxybenzoic acid and bisphenol A with other polymers. The patent describes the unblended copolymer as a "transparent resin having a high heat distortion temperature and excellent mechanical and electrical properties," but states that the copolymer is "hard to process by a conventional processing machine because of the high flow-starting temperature." The '549 patent then reports that blending the copolymers with polycarbonates or aliphatic polyesters corrects the "defects" of both components of the blends, yielding a blend that is more readily processed than the unblended copolymer.
A process for synthesizing copolymers of isophthalic and/or terephthalic acid, 4-hydroxybenzoic acid and bisphenol A is disclosed in U.S. Pat. No. 4,075,173. This patent teaches that the copolymer can readily be made in high molecular weight by first synthesizing a low molecular weight pre-polymer using a melt condensation process and then raising the molecular weight by solid state polymerization. The patent uses a two-step method because bulk polymerization directly to a high molecular weight results in coloration and degradation, which are caused by the high temperatures that are needed to keep the polymer in a molten state as the polymerization proceeds. The patent particularly emphasizes that polymerization of the acetate esters of the phenols and the free acids has the "fatal drawbacks" of coloration and deterioration.
These two patents therefore teach that a copolymer of isophthalic and/or terephthalic acid, 4-hydroxybenzoic acid, and bisphenol A cannot be made in high molecular weight by a direct melt polymerization, especially by polymerization of the acetate esters of the phenols. These patents furthermore suggest that these copolymers cannot be readily processed in the melt using conventional techniques (e.g., injection molding).
There is also one report of a copolymer of terephthalic acid, bisphenol A and 4-hydroxybenzoic acid (J. W. Jackson, Jr., British Polymer Journal, 12, pp. 154-162, December, 1980). This publication reports that incorporation of bisphenol A and terephthalic acid at a 30 mole % level in poly(4-hydroxybenzoate) (i.e., terephthalic acid:bisphenol A:4-hydroxybenzoic acid=30:30:70) eliminates the liquid crystallinity of poly(4-hydroxybenzoate). The publication does not comment on the ease or difficulty of processing the copolymer in the melt, since the publication is directed to liquid crystalline polymers.
Finally, polyarylates made from isophthalic acid and/or terephthalic acid, bisphenol A and about 5 to 10 mole % of 6-hydroxy-2-naphthoic acid have been reported in U.S. Pat. No. 5,023,314 and a related process patent (U.S. Pat. No. 5,109,101). These polymers preferably contain at least about 10% isophthalic acid (as a % of the two diacids). Polymers containing only terephthalic acid, bisphenol A and 6-hydroxy-2-naphthoic acid are not mentioned in the patent, nor is 4-hydroxybenzoic acid mentioned as a potential comonomer.